/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.util.concurrent;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Supplier;
import com.google.common.base.Throwables;
import com.google.common.collect.Lists;
import com.google.common.collect.Queues;
import com.google.common.util.concurrent.ForwardingListenableFuture.SimpleForwardingListenableFuture;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.lang.reflect.InvocationTargetException;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.Delayed;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import javax.annotation.concurrent.GuardedBy;

/**
 * Factory and utility methods for {@link java.util.concurrent.Executor}, {@link ExecutorService},
 * and {@link ThreadFactory}.
 *
 * @author Eric Fellheimer
 * @author Kyle Littlefield
 * @author Justin Mahoney
 * @since 3.0
 */
@GwtCompatible(emulated = true)
public final class MoreExecutors {
    private MoreExecutors() {}

    /**
     * Converts the given ThreadPoolExecutor into an ExecutorService that exits when the application
     * is complete. It does so by using daemon threads and adding a shutdown hook to wait for their
     * completion.
     *
     * <p>
     * This is mainly for fixed thread pools. See {@link Executors#newFixedThreadPool(int)}.
     *
     * @param executor the executor to modify to make sure it exits when the application is finished
     * @param terminationTimeout how long to wait for the executor to finish before terminating the
     *        JVM
     * @param timeUnit unit of time for the time parameter
     * @return an unmodifiable version of the input which will not hang the JVM
     */
    @Beta
    @GwtIncompatible // TODO
    public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor, long terminationTimeout,
            TimeUnit timeUnit) {
        return new Application().getExitingExecutorService(executor, terminationTimeout, timeUnit);
    }

    /**
     * Converts the given ScheduledThreadPoolExecutor into a ScheduledExecutorService that exits
     * when the application is complete. It does so by using daemon threads and adding a shutdown
     * hook to wait for their completion.
     *
     * <p>
     * This is mainly for fixed thread pools. See {@link Executors#newScheduledThreadPool(int)}.
     *
     * @param executor the executor to modify to make sure it exits when the application is finished
     * @param terminationTimeout how long to wait for the executor to finish before terminating the
     *        JVM
     * @param timeUnit unit of time for the time parameter
     * @return an unmodifiable version of the input which will not hang the JVM
     */
    @Beta
    @GwtIncompatible // TODO
    public static ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor,
            long terminationTimeout, TimeUnit timeUnit) {
        return new Application().getExitingScheduledExecutorService(executor, terminationTimeout, timeUnit);
    }

    /**
     * Add a shutdown hook to wait for thread completion in the given {@link ExecutorService
     * service}. This is useful if the given service uses daemon threads, and we want to keep the
     * JVM from exiting immediately on shutdown, instead giving these daemon threads a chance to
     * terminate normally.
     *
     * @param service ExecutorService which uses daemon threads
     * @param terminationTimeout how long to wait for the executor to finish before terminating the
     *        JVM
     * @param timeUnit unit of time for the time parameter
     */
    @Beta
    @GwtIncompatible // TODO
    public static void addDelayedShutdownHook(ExecutorService service, long terminationTimeout, TimeUnit timeUnit) {
        new Application().addDelayedShutdownHook(service, terminationTimeout, timeUnit);
    }

    /**
     * Converts the given ThreadPoolExecutor into an ExecutorService that exits when the application
     * is complete. It does so by using daemon threads and adding a shutdown hook to wait for their
     * completion.
     *
     * <p>
     * This method waits 120 seconds before continuing with JVM termination, even if the executor
     * has not finished its work.
     *
     * <p>
     * This is mainly for fixed thread pools. See {@link Executors#newFixedThreadPool(int)}.
     *
     * @param executor the executor to modify to make sure it exits when the application is finished
     * @return an unmodifiable version of the input which will not hang the JVM
     */
    @Beta
    @GwtIncompatible // concurrency
    public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor) {
        return new Application().getExitingExecutorService(executor);
    }

    /**
     * Converts the given ScheduledThreadPoolExecutor into a ScheduledExecutorService that exits
     * when the application is complete. It does so by using daemon threads and adding a shutdown
     * hook to wait for their completion.
     *
     * <p>
     * This method waits 120 seconds before continuing with JVM termination, even if the executor
     * has not finished its work.
     *
     * <p>
     * This is mainly for fixed thread pools. See {@link Executors#newScheduledThreadPool(int)}.
     *
     * @param executor the executor to modify to make sure it exits when the application is finished
     * @return an unmodifiable version of the input which will not hang the JVM
     */
    @Beta
    @GwtIncompatible // TODO
    public static ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor) {
        return new Application().getExitingScheduledExecutorService(executor);
    }

    /** Represents the current application to register shutdown hooks. */
    @GwtIncompatible // TODO
    @VisibleForTesting
    static class Application {

        final ExecutorService getExitingExecutorService(ThreadPoolExecutor executor, long terminationTimeout,
                TimeUnit timeUnit) {
            useDaemonThreadFactory(executor);
            ExecutorService service = Executors.unconfigurableExecutorService(executor);
            addDelayedShutdownHook(service, terminationTimeout, timeUnit);
            return service;
        }

        final ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor,
                long terminationTimeout, TimeUnit timeUnit) {
            useDaemonThreadFactory(executor);
            ScheduledExecutorService service = Executors.unconfigurableScheduledExecutorService(executor);
            addDelayedShutdownHook(service, terminationTimeout, timeUnit);
            return service;
        }

        final void addDelayedShutdownHook(final ExecutorService service, final long terminationTimeout,
                final TimeUnit timeUnit) {
            checkNotNull(service);
            checkNotNull(timeUnit);
            addShutdownHook(MoreExecutors.newThread("DelayedShutdownHook-for-" + service, new Runnable() {
                @Override
                public void run() {
                    try {
                        // We'd like to log progress and failures that may arise in the
                        // following code, but unfortunately the behavior of logging
                        // is undefined in shutdown hooks.
                        // This is because the logging code installs a shutdown hook of its
                        // own. See Cleaner class inside {@link LogManager}.
                        service.shutdown();
                        service.awaitTermination(terminationTimeout, timeUnit);
                    } catch (InterruptedException ignored) {
                        // We're shutting down anyway, so just ignore.
                    }
                }
            }));
        }

        final ExecutorService getExitingExecutorService(ThreadPoolExecutor executor) {
            return getExitingExecutorService(executor, 120, TimeUnit.SECONDS);
        }

        final ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor) {
            return getExitingScheduledExecutorService(executor, 120, TimeUnit.SECONDS);
        }

        @VisibleForTesting
        void addShutdownHook(Thread hook) {
            Runtime.getRuntime().addShutdownHook(hook);
        }
    }

    @GwtIncompatible // TODO
    private static void useDaemonThreadFactory(ThreadPoolExecutor executor) {
        executor.setThreadFactory(
                new ThreadFactoryBuilder().setDaemon(true).setThreadFactory(executor.getThreadFactory()).build());
    }

    // See newDirectExecutorService javadoc for behavioral notes.
    @GwtIncompatible // TODO
    private static final class DirectExecutorService extends AbstractListeningExecutorService {
        /**
         * Lock used whenever accessing the state variables (runningTasks, shutdown) of the executor
         */
        private final Object lock = new Object();

        /*
         * Conceptually, these two variables describe the executor being in one of three states: -
         * Active: shutdown == false - Shutdown: runningTasks > 0 and shutdown == true - Terminated:
         * runningTasks == 0 and shutdown == true
         */
        @GuardedBy("lock")
        private int runningTasks = 0;

        @GuardedBy("lock")
        private boolean shutdown = false;

        @Override
        public void execute(Runnable command) {
            startTask();
            try {
                command.run();
            } finally {
                endTask();
            }
        }

        @Override
        public boolean isShutdown() {
            synchronized (lock) {
                return shutdown;
            }
        }

        @Override
        public void shutdown() {
            synchronized (lock) {
                shutdown = true;
                if (runningTasks == 0) {
                    lock.notifyAll();
                }
            }
        }

        // See newDirectExecutorService javadoc for unusual behavior of this method.
        @Override
        public List<Runnable> shutdownNow() {
            shutdown();
            return Collections.emptyList();
        }

        @Override
        public boolean isTerminated() {
            synchronized (lock) {
                return shutdown && runningTasks == 0;
            }
        }

        @Override
        public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
            long nanos = unit.toNanos(timeout);
            synchronized (lock) {
                while (true) {
                    if (shutdown && runningTasks == 0) {
                        return true;
                    } else if (nanos <= 0) {
                        return false;
                    } else {
                        long now = System.nanoTime();
                        TimeUnit.NANOSECONDS.timedWait(lock, nanos);
                        nanos -= System.nanoTime() - now; // subtract the actual time we waited
                    }
                }
            }
        }

        /**
         * Checks if the executor has been shut down and increments the running task count.
         *
         * @throws RejectedExecutionException if the executor has been previously shutdown
         */
        private void startTask() {
            synchronized (lock) {
                if (shutdown) {
                    throw new RejectedExecutionException("Executor already shutdown");
                }
                runningTasks++;
            }
        }

        /**
         * Decrements the running task count.
         */
        private void endTask() {
            synchronized (lock) {
                int numRunning = --runningTasks;
                if (numRunning == 0) {
                    lock.notifyAll();
                }
            }
        }
    }

    /**
     * Creates an executor service that runs each task in the thread that invokes
     * {@code execute/submit}, as in {@link CallerRunsPolicy} This applies both to individually
     * submitted tasks and to collections of tasks submitted via {@code invokeAll} or
     * {@code invokeAny}. In the latter case, tasks will run serially on the calling thread. Tasks
     * are run to completion before a {@code Future} is returned to the caller (unless the executor
     * has been shutdown).
     *
     * <p>
     * Although all tasks are immediately executed in the thread that submitted the task, this
     * {@code ExecutorService} imposes a small locking overhead on each task submission in order to
     * implement shutdown and termination behavior.
     *
     * <p>
     * The implementation deviates from the {@code ExecutorService} specification with regards to
     * the {@code shutdownNow} method. First, "best-effort" with regards to canceling running tasks
     * is implemented as "no-effort". No interrupts or other attempts are made to stop threads
     * executing tasks. Second, the returned list will always be empty, as any submitted task is
     * considered to have started execution. This applies also to tasks given to {@code invokeAll}
     * or {@code invokeAny} which are pending serial execution, even the subset of the tasks that
     * have not yet started execution. It is unclear from the {@code ExecutorService} specification
     * if these should be included, and it's much easier to implement the interpretation that they
     * not be. Finally, a call to {@code shutdown} or {@code shutdownNow} may result in concurrent
     * calls to {@code invokeAll/invokeAny} throwing RejectedExecutionException, although a subset
     * of the tasks may already have been executed.
     *
     * @since 18.0 (present as MoreExecutors.sameThreadExecutor() since 10.0)
     */
    @GwtIncompatible // TODO
    public static ListeningExecutorService newDirectExecutorService() {
        return new DirectExecutorService();
    }

    /**
     * Returns an {@link Executor} that runs each task in the thread that invokes
     * {@link Executor#execute execute}, as in {@link CallerRunsPolicy}.
     *
     * <p>
     * This instance is equivalent to:
     * 
     * <pre>
     * {
     *     &#64;code
     *     final class DirectExecutor implements Executor {
     *         public void execute(Runnable r) {
     *             r.run();
     *         }
     *     }
     * }
     * </pre>
     *
     * <p>
     * This should be preferred to {@link #newDirectExecutorService()} because implementing the
     * {@link ExecutorService} subinterface necessitates significant performance overhead.
     *
     * @since 18.0
     */
    public static Executor directExecutor() {
        return DirectExecutor.INSTANCE;
    }

    /** See {@link #directExecutor} for behavioral notes. */
    private enum DirectExecutor implements Executor {
        INSTANCE;

        @Override
        public void execute(Runnable command) {
            command.run();
        }

        @Override
        public String toString() {
            return "MoreExecutors.directExecutor()";
        }
    }

    /**
     * Creates an {@link ExecutorService} whose {@code submit} and {@code
     * invokeAll} methods submit {@link ListenableFutureTask} instances to the given delegate
     * executor. Those methods, as well as {@code execute} and {@code invokeAny}, are implemented in
     * terms of calls to {@code
     * delegate.execute}. All other methods are forwarded unchanged to the delegate. This implies
     * that the returned {@code ListeningExecutorService} never calls the delegate's {@code submit},
     * {@code invokeAll}, and {@code
     * invokeAny} methods, so any special handling of tasks must be implemented in the delegate's
     * {@code execute} method or by wrapping the returned {@code
     * ListeningExecutorService}.
     *
     * <p>
     * If the delegate executor was already an instance of {@code
     * ListeningExecutorService}, it is returned untouched, and the rest of this documentation does
     * not apply.
     *
     * @since 10.0
     */
    @GwtIncompatible // TODO
    public static ListeningExecutorService listeningDecorator(ExecutorService delegate) {
        return (delegate instanceof ListeningExecutorService) ? (ListeningExecutorService) delegate
                : (delegate instanceof ScheduledExecutorService)
                        ? new ScheduledListeningDecorator((ScheduledExecutorService) delegate)
                        : new ListeningDecorator(delegate);
    }

    /**
     * Creates a {@link ScheduledExecutorService} whose {@code submit} and {@code invokeAll} methods
     * submit {@link ListenableFutureTask} instances to the given delegate executor. Those methods,
     * as well as {@code execute} and {@code invokeAny}, are implemented in terms of calls to {@code
     * delegate.execute}. All other methods are forwarded unchanged to the delegate. This implies
     * that the returned {@code ListeningScheduledExecutorService} never calls the delegate's {@code
     * submit}, {@code invokeAll}, and {@code invokeAny} methods, so any special handling of tasks
     * must be implemented in the delegate's {@code execute} method or by wrapping the returned
     * {@code
     * ListeningScheduledExecutorService}.
     *
     * <p>
     * If the delegate executor was already an instance of {@code
     * ListeningScheduledExecutorService}, it is returned untouched, and the rest of this
     * documentation does not apply.
     *
     * @since 10.0
     */
    @GwtIncompatible // TODO
    public static ListeningScheduledExecutorService listeningDecorator(ScheduledExecutorService delegate) {
        return (delegate instanceof ListeningScheduledExecutorService) ? (ListeningScheduledExecutorService) delegate
                : new ScheduledListeningDecorator(delegate);
    }

    @GwtIncompatible // TODO
    private static class ListeningDecorator extends AbstractListeningExecutorService {
        private final ExecutorService delegate;

        ListeningDecorator(ExecutorService delegate) {
            this.delegate = checkNotNull(delegate);
        }

        @Override
        public final boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException {
            return delegate.awaitTermination(timeout, unit);
        }

        @Override
        public final boolean isShutdown() {
            return delegate.isShutdown();
        }

        @Override
        public final boolean isTerminated() {
            return delegate.isTerminated();
        }

        @Override
        public final void shutdown() {
            delegate.shutdown();
        }

        @Override
        public final List<Runnable> shutdownNow() {
            return delegate.shutdownNow();
        }

        @Override
        public final void execute(Runnable command) {
            delegate.execute(command);
        }
    }

    @GwtIncompatible // TODO
    private static final class ScheduledListeningDecorator extends ListeningDecorator
            implements ListeningScheduledExecutorService {
        @SuppressWarnings("hiding")
        final ScheduledExecutorService delegate;

        ScheduledListeningDecorator(ScheduledExecutorService delegate) {
            super(delegate);
            this.delegate = checkNotNull(delegate);
        }

        @Override
        public ListenableScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) {
            TrustedListenableFutureTask<Void> task = TrustedListenableFutureTask.create(command, null);
            ScheduledFuture<?> scheduled = delegate.schedule(task, delay, unit);
            return new ListenableScheduledTask<Void>(task, scheduled);
        }

        @Override
        public <V> ListenableScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) {
            TrustedListenableFutureTask<V> task = TrustedListenableFutureTask.create(callable);
            ScheduledFuture<?> scheduled = delegate.schedule(task, delay, unit);
            return new ListenableScheduledTask<V>(task, scheduled);
        }

        @Override
        public ListenableScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period,
                TimeUnit unit) {
            NeverSuccessfulListenableFutureTask task = new NeverSuccessfulListenableFutureTask(command);
            ScheduledFuture<?> scheduled = delegate.scheduleAtFixedRate(task, initialDelay, period, unit);
            return new ListenableScheduledTask<Void>(task, scheduled);
        }

        @Override
        public ListenableScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay,
                TimeUnit unit) {
            NeverSuccessfulListenableFutureTask task = new NeverSuccessfulListenableFutureTask(command);
            ScheduledFuture<?> scheduled = delegate.scheduleWithFixedDelay(task, initialDelay, delay, unit);
            return new ListenableScheduledTask<Void>(task, scheduled);
        }

        private static final class ListenableScheduledTask<V> extends SimpleForwardingListenableFuture<V>
                implements ListenableScheduledFuture<V> {

            private final ScheduledFuture<?> scheduledDelegate;

            public ListenableScheduledTask(ListenableFuture<V> listenableDelegate,
                    ScheduledFuture<?> scheduledDelegate) {
                super(listenableDelegate);
                this.scheduledDelegate = scheduledDelegate;
            }

            @Override
            public boolean cancel(boolean mayInterruptIfRunning) {
                boolean cancelled = super.cancel(mayInterruptIfRunning);
                if (cancelled) {
                    // Unless it is cancelled, the delegate may continue being scheduled
                    scheduledDelegate.cancel(mayInterruptIfRunning);

                    // TODO(user): Cancel "this" if "scheduledDelegate" is cancelled.
                }
                return cancelled;
            }

            @Override
            public long getDelay(TimeUnit unit) {
                return scheduledDelegate.getDelay(unit);
            }

            @Override
            public int compareTo(Delayed other) {
                return scheduledDelegate.compareTo(other);
            }
        }

        @GwtIncompatible // TODO
        private static final class NeverSuccessfulListenableFutureTask extends AbstractFuture<Void>
                implements Runnable {
            private final Runnable delegate;

            public NeverSuccessfulListenableFutureTask(Runnable delegate) {
                this.delegate = checkNotNull(delegate);
            }

            @Override
            public void run() {
                try {
                    delegate.run();
                } catch (Throwable t) {
                    setException(t);
                    throw Throwables.propagate(t);
                }
            }
        }
    }

    /*
     * This following method is a modified version of one found in
     * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/test/tck/AbstractExecutorServiceTest.
     * java?revision=1.30 which contained the following notice:
     *
     * Written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to
     * the public domain, as explained at http://creativecommons.org/publicdomain/zero/1.0/
     *
     * Other contributors include Andrew Wright, Jeffrey Hayes, Pat Fisher, Mike Judd.
     */

    /**
     * An implementation of {@link ExecutorService#invokeAny} for {@link ListeningExecutorService}
     * implementations.
     */
    @GwtIncompatible
    static <T> T invokeAnyImpl(ListeningExecutorService executorService, Collection<? extends Callable<T>> tasks,
            boolean timed, long timeout, TimeUnit unit)
            throws InterruptedException, ExecutionException, TimeoutException {
        checkNotNull(executorService);
        checkNotNull(unit);
        int ntasks = tasks.size();
        checkArgument(ntasks > 0);
        List<Future<T>> futures = Lists.newArrayListWithCapacity(ntasks);
        BlockingQueue<Future<T>> futureQueue = Queues.newLinkedBlockingQueue();
        long timeoutNanos = unit.toNanos(timeout);

        // For efficiency, especially in executors with limited
        // parallelism, check to see if previously submitted tasks are
        // done before submitting more of them. This interleaving
        // plus the exception mechanics account for messiness of main
        // loop.

        try {
            // Record exceptions so that if we fail to obtain any
            // result, we can throw the last exception we got.
            ExecutionException ee = null;
            long lastTime = timed ? System.nanoTime() : 0;
            Iterator<? extends Callable<T>> it = tasks.iterator();

            futures.add(submitAndAddQueueListener(executorService, it.next(), futureQueue));
            --ntasks;
            int active = 1;

            while (true) {
                Future<T> f = futureQueue.poll();
                if (f == null) {
                    if (ntasks > 0) {
                        --ntasks;
                        futures.add(submitAndAddQueueListener(executorService, it.next(), futureQueue));
                        ++active;
                    } else if (active == 0) {
                        break;
                    } else if (timed) {
                        f = futureQueue.poll(timeoutNanos, TimeUnit.NANOSECONDS);
                        if (f == null) {
                            throw new TimeoutException();
                        }
                        long now = System.nanoTime();
                        timeoutNanos -= now - lastTime;
                        lastTime = now;
                    } else {
                        f = futureQueue.take();
                    }
                }
                if (f != null) {
                    --active;
                    try {
                        return f.get();
                    } catch (ExecutionException eex) {
                        ee = eex;
                    } catch (RuntimeException rex) {
                        ee = new ExecutionException(rex);
                    }
                }
            }

            if (ee == null) {
                ee = new ExecutionException(null);
            }
            throw ee;
        } finally {
            for (Future<T> f : futures) {
                f.cancel(true);
            }
        }
    }

    /**
     * Submits the task and adds a listener that adds the future to {@code queue} when it completes.
     */
    @GwtIncompatible // TODO
    private static <T> ListenableFuture<T> submitAndAddQueueListener(ListeningExecutorService executorService,
            Callable<T> task, final BlockingQueue<Future<T>> queue) {
        final ListenableFuture<T> future = executorService.submit(task);
        future.addListener(new Runnable() {
            @Override
            public void run() {
                queue.add(future);
            }
        }, directExecutor());
        return future;
    }

    /**
     * Returns a default thread factory used to create new threads.
     *
     * <p>
     * On AppEngine, returns {@code ThreadManager.currentRequestThreadFactory()}. Otherwise, returns
     * {@link Executors#defaultThreadFactory()}.
     *
     * @since 14.0
     */
    @Beta
    @GwtIncompatible // concurrency
    public static ThreadFactory platformThreadFactory() {
        if (!isAppEngine()) {
            return Executors.defaultThreadFactory();
        }
        try {
            return (ThreadFactory) Class.forName("com.google.appengine.api.ThreadManager")
                    .getMethod("currentRequestThreadFactory").invoke(null);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (NoSuchMethodException e) {
            throw new RuntimeException("Couldn't invoke ThreadManager.currentRequestThreadFactory", e);
        } catch (InvocationTargetException e) {
            throw Throwables.propagate(e.getCause());
        }
    }

    @GwtIncompatible // TODO
    private static boolean isAppEngine() {
        if (System.getProperty("com.google.appengine.runtime.environment") == null) {
            return false;
        }
        try {
            // If the current environment is null, we're not inside AppEngine.
            return Class.forName("com.google.apphosting.api.ApiProxy").getMethod("getCurrentEnvironment")
                    .invoke(null) != null;
        } catch (ClassNotFoundException e) {
            // If ApiProxy doesn't exist, we're not on AppEngine at all.
            return false;
        } catch (InvocationTargetException e) {
            // If ApiProxy throws an exception, we're not in a proper AppEngine environment.
            return false;
        } catch (IllegalAccessException e) {
            // If the method isn't accessible, we're not on a supported version of AppEngine;
            return false;
        } catch (NoSuchMethodException e) {
            // If the method doesn't exist, we're not on a supported version of AppEngine;
            return false;
        }
    }

    /**
     * Creates a thread using {@link #platformThreadFactory}, and sets its name to {@code name}
     * unless changing the name is forbidden by the security manager.
     */
    @GwtIncompatible // concurrency
    static Thread newThread(String name, Runnable runnable) {
        checkNotNull(name);
        checkNotNull(runnable);
        Thread result = platformThreadFactory().newThread(runnable);
        try {
            result.setName(name);
        } catch (SecurityException e) {
            // OK if we can't set the name in this environment.
        }
        return result;
    }

    // TODO(lukes): provide overloads for ListeningExecutorService?
    // ListeningScheduledExecutorService?
    // TODO(lukes): provide overloads that take constant strings? Function<Runnable, String>s to
    // calculate names?

    /**
     * Creates an {@link Executor} that renames the {@link Thread threads} that its tasks run in.
     *
     * <p>
     * The names are retrieved from the {@code nameSupplier} on the thread that is being renamed
     * right before each task is run. The renaming is best effort, if a {@link SecurityManager}
     * prevents the renaming then it will be skipped but the tasks will still execute.
     *
     *
     * @param executor The executor to decorate
     * @param nameSupplier The source of names for each task
     */
    @GwtIncompatible // concurrency
    static Executor renamingDecorator(final Executor executor, final Supplier<String> nameSupplier) {
        checkNotNull(executor);
        checkNotNull(nameSupplier);
        if (isAppEngine()) {
            // AppEngine doesn't support thread renaming, so don't even try
            return executor;
        }
        return new Executor() {
            @Override
            public void execute(Runnable command) {
                executor.execute(Callables.threadRenaming(command, nameSupplier));
            }
        };
    }

    /**
     * Creates an {@link ExecutorService} that renames the {@link Thread threads} that its tasks run
     * in.
     *
     * <p>
     * The names are retrieved from the {@code nameSupplier} on the thread that is being renamed
     * right before each task is run. The renaming is best effort, if a {@link SecurityManager}
     * prevents the renaming then it will be skipped but the tasks will still execute.
     *
     *
     * @param service The executor to decorate
     * @param nameSupplier The source of names for each task
     */
    @GwtIncompatible // concurrency
    static ExecutorService renamingDecorator(final ExecutorService service, final Supplier<String> nameSupplier) {
        checkNotNull(service);
        checkNotNull(nameSupplier);
        if (isAppEngine()) {
            // AppEngine doesn't support thread renaming, so don't even try.
            return service;
        }
        return new WrappingExecutorService(service) {
            @Override
            protected <T> Callable<T> wrapTask(Callable<T> callable) {
                return Callables.threadRenaming(callable, nameSupplier);
            }

            @Override
            protected Runnable wrapTask(Runnable command) {
                return Callables.threadRenaming(command, nameSupplier);
            }
        };
    }

    /**
     * Creates a {@link ScheduledExecutorService} that renames the {@link Thread threads} that its
     * tasks run in.
     *
     * <p>
     * The names are retrieved from the {@code nameSupplier} on the thread that is being renamed
     * right before each task is run. The renaming is best effort, if a {@link SecurityManager}
     * prevents the renaming then it will be skipped but the tasks will still execute.
     *
     *
     * @param service The executor to decorate
     * @param nameSupplier The source of names for each task
     */
    @GwtIncompatible // concurrency
    static ScheduledExecutorService renamingDecorator(final ScheduledExecutorService service,
            final Supplier<String> nameSupplier) {
        checkNotNull(service);
        checkNotNull(nameSupplier);
        if (isAppEngine()) {
            // AppEngine doesn't support thread renaming, so don't even try.
            return service;
        }
        return new WrappingScheduledExecutorService(service) {
            @Override
            protected <T> Callable<T> wrapTask(Callable<T> callable) {
                return Callables.threadRenaming(callable, nameSupplier);
            }

            @Override
            protected Runnable wrapTask(Runnable command) {
                return Callables.threadRenaming(command, nameSupplier);
            }
        };
    }

    /**
     * Shuts down the given executor service gradually, first disabling new submissions and later,
     * if necessary, cancelling remaining tasks.
     *
     * <p>
     * The method takes the following steps:
     * <ol>
     * <li>calls {@link ExecutorService#shutdown()}, disabling acceptance of new submitted tasks.
     * <li>awaits executor service termination for half of the specified timeout.
     * <li>if the timeout expires, it calls {@link ExecutorService#shutdownNow()}, cancelling
     * pending tasks and interrupting running tasks.
     * <li>awaits executor service termination for the other half of the specified timeout.
     * </ol>
     *
     * <p>
     * If, at any step of the process, the calling thread is interrupted, the method calls
     * {@link ExecutorService#shutdownNow()} and returns.
     *
     * @param service the {@code ExecutorService} to shut down
     * @param timeout the maximum time to wait for the {@code ExecutorService} to terminate
     * @param unit the time unit of the timeout argument
     * @return {@code true} if the {@code ExecutorService} was terminated successfully,
     *         {@code false} if the call timed out or was interrupted
     * @since 17.0
     */
    @Beta
    @CanIgnoreReturnValue
    @GwtIncompatible // concurrency
    public static boolean shutdownAndAwaitTermination(ExecutorService service, long timeout, TimeUnit unit) {
        long halfTimeoutNanos = unit.toNanos(timeout) / 2;
        // Disable new tasks from being submitted
        service.shutdown();
        try {
            // Wait for half the duration of the timeout for existing tasks to terminate
            if (!service.awaitTermination(halfTimeoutNanos, TimeUnit.NANOSECONDS)) {
                // Cancel currently executing tasks
                service.shutdownNow();
                // Wait the other half of the timeout for tasks to respond to being cancelled
                service.awaitTermination(halfTimeoutNanos, TimeUnit.NANOSECONDS);
            }
        } catch (InterruptedException ie) {
            // Preserve interrupt status
            Thread.currentThread().interrupt();
            // (Re-)Cancel if current thread also interrupted
            service.shutdownNow();
        }
        return service.isTerminated();
    }

    /**
     * Returns an Executor that will propagate {@link RejectedExecutionException} from the delegate
     * executor to the given {@code future}.
     *
     * <p>
     * Note, the returned executor can only be used once.
     */
    static Executor rejectionPropagatingExecutor(final Executor delegate, final AbstractFuture<?> future) {
        checkNotNull(delegate);
        checkNotNull(future);
        if (delegate == directExecutor()) {
            // directExecutor() cannot throw RejectedExecutionException
            return delegate;
        }
        return new Executor() {
            volatile boolean thrownFromDelegate = true;

            @Override
            public void execute(final Runnable command) {
                try {
                    delegate.execute(new Runnable() {
                        @Override
                        public void run() {
                            thrownFromDelegate = false;
                            command.run();
                        }
                    });
                } catch (RejectedExecutionException e) {
                    if (thrownFromDelegate) {
                        // wrap exception?
                        future.setException(e);
                    }
                    // otherwise it must have been thrown from a transitive call and the delegate
                    // runnable
                    // should have handled it.
                }
            }
        };
    }
}
